Project Summary

Project Abstract:
Evidence in the literature indicates that the stiffness of the asphalt binder increases and ductility of the binder decreases with oxidative aging. Typically for unmodified asphalt binders an increase in stiffness or decrease in ductility is regarded as detrimental to the fatigue cracking or fracture resistance of the asphalt binder. However, fundamentally the stiffness of the binder and its strength are two different attributes that may not necessarily be related to each other. There is very little information in the literature that relates the fatigue cracking resistance or strength of the asphalt binder to the extent of oxidative aging. This information is not only important to assess the durability and cracking life of asphalt pavements but is also very important in the context of recycled asphalt pavements (RAP). The use of RAP not only reduces the waste produced from milling and removing asphalt pavement layers at the end of their service life, but also reduces the amount of asphalt required for construction of new roadways. In an effort to improve sustainable practices associated with pavement constructions, state DOTs have been gradually increasing the allowable percentage of RAP in new asphalt mixtures over the last two decades. However, the asphalt binder in RAP is highly oxidized and is deemed to susceptible to load related fatigue cracking. The focus of this study is to investigate the affect of asphalt binder oxidation on its fatigue cracking performance. To this end, fatigue cracking resistance of an asphalt binder will be measured at different levels of aging using an standardized glass bead composite. The glass bead composite simulates the stress state that asphalt binders experience in the field, while it excludes aggregate-asphalt binder interactions. Furthermore, this research investigates the effect of aging on fracture properties of an asphalt binder by conducting monotonic fracture tests using a poker chip test geometry. This study also investigates the effect rejuvenators on the fatigue performance of RAP. The findings from this study will provide a better understanding of the RAP long term performance, and also provides an analytic approach for choosing RAP mixture proportions.

Project Objectives:
The main objective of this study is to evaluate the affect of aging and use of RAP on the fatigue cracking resistance of asphalt binders at an intermediate temperature. The PI has specialized in measuring the fatigue and healing properties of asphaltic materials using different experimental and analytical tools. To the best of the PIs knowledge there has not been any study to measure these properties at different levels of aging using and stress states similar to what asphalt binders experience in the field. In this study, we will use a standardized glass bead composite to measure the fatigue and healing properties of an asphalt binder at different levels of aging. The glass bead composite simulates the stress state that asphalt binders experience within HMA, while it excludes aggregate-asphalt binder interactions. To age asphalt binders in the laboratory in different levels, two different aging devices will be use: rolling thin film oven (RTFO), and pressure aging vessel (PAV). The RTFO simulates the short-term aging due to construction, and the PAV simulates the long-term aging due to construction and service. Furthermore, to simulate the extended aging, the PAV-aged asphalt binder will be further aged using PAV for the second and third times. Finally to simulate the HMA containing RAP, the RTFO-aged binder will be mixed with the double PAV-aged binder. After aging the binder and fabricating standardized glass bead composites, the mechanical (undamaged, fatigue, and healing) properties of the asphalt binder will be measured. This will include the mechanical properties of asphalt binder at different levels of aging and also mixtures with different percentage of RAP. To analyze the fatigue test results, we will use continuum damage mechanics. This study will also investigate the effect rejuvenators on the fatigue performance of RAP mixtures.

The second objective of this study is to measure the facture properties of the asphalt binder at different level of aging or RAP content. The poker chip geometry will be used to measure the fracture properties of the aged-virgin asphalt binder composite at an intermediate temperature.

This study also quantifies the amount of aging, and seeks a correlation between amount of aging and the measured mechanical properties. Two-dimensional FTIR mapping will be used to quantify the amount of aging (by measuring the carbonyl area). The FTIR mapping will also provide a tool to examine the extent of co-mingling between the virgin binder (here RTFO-aged) and RAP binder (here double PAV-aged).

The findings from this study will provide a better understanding of the RAP long term performance, and also provides an analytic approach for choosing RAP mixture proportions.

Task Descriptions:

Task 1. Literature reviewA detailed literature review will be conducted with emphasis on the following three areas.

1. Methods to measure the oxidation and aging in asphalt binders such as FTIR.
2. Methods to measure the mechanical properties of asphalt binders such as

Task 2. Preparation of testing materialsTest specimens will be fabricated using asphalt binder aged in the laboratory to represent different levels of oxidation. This will be accomplished using two different aging devices: rolling thin film oven (RTFO), and pressure aging vessel (PAV). The RTFO simulates the short-term aging due to construction, and the PAV simulates the long-term aging due to construction and service. Furthermore, to simulate the aging during very long periods of time, the PAV-aged asphalt binder will be further aged using PAV for the second and third times. Finally, to simulate the HMA containing RAP, the RTFO-aged binder will be mixed with the double PAV-aged binder.

Task 3. Quantifying the extent of oxidation (FTIR)This study quantifies the amount of aging after each RTFO or PAV aging. Two-dimensional FTIR mapping will be used to quantify the amount of aging (by measuring the carbonyl area). This data will be later used to find a correlation between the amount of aging and the measured mechanical properties. The FTIR mapping will also provide a tool to examine the extent of co-mingling between the virgin binder (here RTFO-aged) and RAP binder (here double PAV-aged).

Task 4. Fatigue test using a standardized composite
After aging the binder, the mechanical properties (viscoelastic and fatigue cracking resistance) of the asphalt binder will be measured at an intermediate temperature. This will include the mechanical properties of asphalt binder at different levels of aging and also mixtures with different percentage of RAP. To measure the fatigue and healing properties of an asphalt binder, we will fabricate standardized asphalt binder-glass bead composite. The glass bead composite simulates the stress state that asphalt binders experience in mixtures, while it excludes aggregate-asphalt binder interactions. Then, we will use a dynamic shear rheometer (DSR) to apply cyclic loads in shear, and to measure fatigue-cracking resistance of the asphalt binder. The test results will be analyzed using continuum damage mechanics. This will allow testing materials at different stress or strain levels, and comparing their performance under a similar loading condition.

Task 5. Fracture test using poker chip test geometry
In this study we will also measure the facture properties of the asphalt binder at different levels of aging or RAP content. The poker chip geometry will be used to measure the fracture properties of the aged-virgin asphalt binder composite at an intermediate temperature. We will use an Instron tension-compression instrument to conduct monotonic fracture tests in tension. Using the concept of pseudo stress and strain we will be able to conduct the monotonic tests at different displacement rates, and gain a better precision in measurements.

Task 6. Documentation of the research findings
A final report documenting the detailed procedures, results, conclusions and recommendations will be prepared at the end of the project. In addition, the results and findings from this research study will also be documented and submitted for consideration as a journal publication.

Implementation of Research Outcomes:The use of reclaimed asphalt pavement (RAP) in asphalt mixes has significantly increased over the last few years. However, the long-term impact of using high percentages of RAP is not clear. Also, mixes with RAP are often regarded as prone to fatigue cracking. The results from this study indicate that it is possible to carefully design mixes with high RAP content without compromising on durability.

Products developed by this research:

Test method developed: The research study has led to the development of a test method to evaluate fatigue cracking resistance of asphalt binders in a more realistic manner. The information has been shared with other Universities and personnel from the TxDOT. Future work using this test method is anticipated.

Journal article in preparation:Influence of Aging in Asphalt Binders on its Fatigue Cracking Resistance in the Context of RAP Mixes, Amit Bhasin, to be submitted to the ASCE Journal of Materials in Civil Engineering.

Impacts/Benefits of Implementation:One of the findings from this study is that the use of reclaimed asphalt pavement in new mixes does not necessarily negatively impact the fatigue cracking resistance of an asphalt mixture. To this end, we believe that this research is a small but critical step to educate pavement engineers on the appropriate design and use of materials.